This invention relates to antennas and more particularly to an improved antenna array capable of producing omnidirectional radiation over an octave bandwidth.
Certain applications of radio frequency communications such as ground station transceivers require an antenna having a horizontally polarized omnidirectional radiation pattern and a voltage standing wave ratio (VSWR) value of 2:1 or lower over an entire octave frequency bandwidth. Such performance is not obtainable with prior art antennas.
One prior art antenna, the Lindenblad antenna described by G. H. Brown and O. M. Woodward, Jr. in "Circularly-Polarized Omnidirectional Antenna", RCA Review, Volume VIII, No. 2; June 1947, pp. 259-269, consists of four half-wave dipole elements fed from a centrally located coaxially line feeder and oriented to lie in a common plane in the form of a square pattern. The dipoles are individually fed from a two-wire transmission line. Four of these lines extend radially from the feeder line to the inputs of the dipoles. The characteristic impedance of the line is selected to optimize the impedance match between the dipoles and the coaxial feed line. While this antenna provides good horizontally polarized radiation patterns over an octave bandwidth, attempts to achieve an impedance match with a VSWR of 2:1 or lower over the same bandwidths have not been successful.
Other approaches exist whereby horizontally polarized radiation patterns are produced. Some of these include different versions of the turnstile antenna, slotted cylinders, the circular-loop antenna, clover leaf antenna, triangular-loop antenna, and square-loop or Alford-type antenna. All of these antennas, without exception, are narrow-band devices in the sense that the impedance bandwidth covers less than an octave.
Still another approach consists of a log-periodic array of turnstile elements. This antenna produces horizontally polarized omnidirectional radiation patterns over an octave bandwidth and even a wider bandwidth if the antenna elements are properly sized. However, the problem with the antenna, like the others, is that its input impedance match has a VSWR greater than 2:1 over an octave bandwidth.